If any of the owners or reputed owners of land in said district are nonresidents of the county or counties in which the proposed district lies, the petition shall be accompanied by an affidavit giving the names and post-office addresses of such nonresidents, if such are known, and if such are unknown, shall state that upon diligent inquiry their names or post-office addresses (whichever may be the fact) cannot be ascertained. The clerk of the court shall mail a copy of the notice aforesaid to each of said nonresidents whose post-office address is known, within six days after the publication of the same.